The study of sound propagation with super-directivity is important for applications ranging from medical imaging, underwater communication, ultrasonic imaging through opaque fluids, to nondestructive testing. In addition, unidirectional and broadband communication using collimated high-frequency sonar ultrasound beams (200 kHz-1 MHz) through barrier walls is necessary for tamper-proof operation of underwater sensitive devices at ocean bottom that can send information from the inside in a beam but ultrasound signals cannot penetrate the device wall in the opposite direction. Simple sound-proofing walls block sound waves from both directions and are not usable for this purpose.
The oil and gas industry makes extensive use of both unmanned underwater vehicles (UUV) and Remotely Operated Vehicles (ROV). ROVs are underwater robots that allow the controller to be located on surface but are connected via an umbilical link that houses communication cables. As more advanced sensing and monitoring devices are developed that are deployed on sea bed or under ocean, it will become increasingly important to protect such communication and make these devices secure and tamper-proof.
The feasibility of an acoustic rectifier consisting of two segments: a sonic crystal (alternating of water and glass), and a nonlinear medium produced from a microbubble suspension has been demonstrated. See, e.g., B. Liang et al. in “An acoustic rectifier,” Nature Materials 2010; 9(12): 989-92. The sonic crystal was designed to behave as an acoustic filter, and sound of frequency ω enters the nonlinear medium first and produces harmonics 2ω that passes through the subsequent sonic crystal, SC, acoustic filter, but blocks the original signal. The pass bands were centered on 1.8 and 2 MHz with a width of approximately 150 kHz for each band. However, when impinging on the opposite side, the original frequency is blocked because of a band gap at that frequency. Another apparatus for acoustic rectification has been demonstrated. See, e.g., N. Boechler et al. in “Bifurcation-based acoustic switching and rectification”, Nature Materials, 2011; 10(9): 665-8, where the interaction of periodicity, nonlinearity, and asymmetry in a granular crystal, are utilized, and which includes a statically compressed, one-dimensional array of particles and a light mass defect near a boundary. This apparatus was demonstrated for very low-frequency (<15 kHz) sound transmission.
A sonic crystal based on a shaped array of scatterer—steel square-prism columns has also been reported. See, e.g., X.-F. Li et al. in “Tunable Unidirectional Sound Propagation through a Sonic-Crystal-Based Acoustic Diode,” Physical Review Letters, 2011; 106(8): 084301. In that apparatus, the sonic crystal achieves unidirectional flow by means of saw-tooth spatial asymmetry in the arrangement of columns. However, the sound waves exiting that apparatus are not parallel to those entering, and the transmitted sound waves are narrow band and at low frequencies (<50 KHz). Tunability can be achieved by mechanical manipulation of the rods.